1. Field of the Invention
The present invention relates generally to a torpedo seeker head, and more particularly to a torpedo seeker head having directional detection independent of frequency.
2. Description of the Background Art
Acoustic (sound) waves have long been used as a method of detecting objects underwater. Because acoustic waves are the type of waves that propagate best through water, they are the choice for applications such as underwater warfare. Sonar (i.e., Sound Navigation And Ranging), is an application of acoustic waves wherein direction and distance to a target may be obtained through the detection of reflected acoustic waves.
Sonar may be of two types, active or passive. Active sonar emits acoustic waves toward a target and picks up reflected waves to determine direction and distance. Passive sonar does not emit any acoustic waves, but only picks up acoustic waves emitted by the target. Passive sonar therefore has an advantage in that it is less likely to give away its own location. Passive sonar is often used when it is desired that the device not transmit any acoustic waves that might be used by the target to locate or track the emitting device, or even alert the target to the presence of the emitting device.
Sonar functions to pickup underwater acoustic waves through the use of a transducer called a hydrophone. The hydrophone is capable of converting received acoustic waves into electrical signals that can be analyzed.
Sonar has practical application in the use of guidance of unmanned weapons, such as a torpedo. A torpedo is essentially a warhead attached to a propulsion system and a guidance system. Without an effective guidance system, a torpedo is a blind missile. A sonar guidance system in the form of a seeker head is capable of detecting a target and guiding the torpedo to the target. The seeker head is capable of detecting target sound, whether it be reflected sound or sound emitted by the target (such as propulsion noise generated by the target).
The related art seeker head array is generally located in the nose area of the torpedo 100, such as shown in the related art torpedo of FIG. 1. The seeker head is commonly accompanied by amplifying and processing circuits. One example of an amplifying and processing configuration is given in U.S. Pat. No. 3,987,404 to Woodruff, incorporated herein by reference. The torpedo 100 can thereby track a target.
Multiple hydrophone elements, typically configured in an array, are used in these acoustic torpedo seeker heads to observe phase differences. Many array configurations are possible. Perhaps most common are square arrays consisting of 4 or 5 hydrophones on a side, for a total of either 16 or 25 hydrophones. FIG. 1 shows a cut-away of a typical torpedo 100, illustrating a typical hydrophone array 103 of the related art. Directional hydrophones may be used for this purpose, but the directional aspects of the hydrophones are not used in the related art torpedo seeker heads. The seeker heads known in the art use only the phase characteristic of received sound waves.
Acoustic torpedo seeker heads known in the related art observe differences in phase of the incoming acoustic signal to determine direction to the target. For example, if the phase angle of the hydrophone elements on the right-hand side of the array is ahead of the phase angle of those on the left-hand side, the seeker head calculates that the target lies to the right of the axis of the seeker head. Conversely, if the phase angle of the hydrophone elements of the left hand side, upper side, or lower side are ahead of the phase angle of the hydrophones on the opposite side of the array, the derived look angle (the discerned direction of the target relative to the axis of the torpedo) indicates that the target lies to the left, above, or below the seeker head, respectively. The greater the difference in phase angle, the greater the angle between the direction to the acoustic source and the axis of the seeker head (this angle is known as the "look angle").
The use of phase differences is illustrated in FIG. 2, which shows acoustic waves of different frequencies. Wave Q is a relatively low frequency wave, while wave R is a relatively high frequency wave. It can be seen from the figure that wave R varies a significant amount of its cycle over the distance P, while wave Q does not vary significantly. Accordingly, wave R could be used to more accurately sense small distance variations of the proportion P.
From FIG. 1, it can be seen that because the acoustic wave source A is located away from the torpedo axis B, the distance to the hydrophone D is greater than the distance to the hydrophone E, so that accordingly, a phase difference exists in the waveform W.sub.1 received by hydrophone E and the waveform W.sub.2 received by hydrophone D. This phase difference can be used to find the look angle . Ideally, in order to obtain an optimum phase difference, the hydrophones would be located far apart in order to have a measurable phase difference for lower frequency acoustic waves.
Because hydrophones in seeker heads known in the related art are relatively closely spaced due to torpedo size constraints, the use of phase differences to measure the look angle must necessarily be limited to a high frequency band. This means that if the seeker head is operating in an active mode, the carrier frequency of the emitted signal must be relatively high for good directionality to be obtained. It also means that if the seeker head is operating in the passive mode, good directionality cannot be obtained on the propulsion lines of the target acoustic signature, since propulsion lines (i.e., propeller noise, etc.), characteristically occur at low frequencies.
Therefore, the inability of related art torpedo seeker heads to reliably detect a direction to a low frequency acoustic source leads to several drawbacks. First, a related art seeker head is complex and costly to purchase and maintain. Second, the related art seeker head encounters a high characteristic of sea attenuation due to the use of high frequencies. Third, target propulsion lines usually occur at low frequencies. Fourth, torpedo ownship noise is generated at high frequencies, as the torpedo propeller blades are small and must rotate rapidly.
Torpedo seeker heads in the related art which rely on high operating frequencies have an additional drawback in that their effectiveness is further reduced by the use of anechoic coatings on targets. Anechoic coatings are coatings that tend to absorb incoming waves in order to reduce a reflected signal. Anechoic coatings are highly effective at frequencies commonly used by related art torpedo seeker heads despite the fact that they are thick, heavy, and expensive. They are much less effective at lower frequencies. Anechoic coatings are unlikely to become effective at low frequencies because of the penalty of prohibitively greater coating thickness and hence cost.
Therefore, there remains a need in the art for a torpedo seeker head having directional detection independent of frequency.